More than 2 million third-party vendors supply around 40% of all of items sold on Amazon, Wired reports. These items are shipped into warehouses all around the world that are just like the Phoenix fulfillment center.

They come in on trucks and are neatly stacked on shelves according to bar codes like books in a library. Some shelves measure as small as one-tenth of an inch wide. When the orders come in, Amazon finds the products according to their bar code, boxes them up, and ships them out to customers, Wired says.

"Most consumers don’t even realize third parties are involved. They just trust Amazon so much that they don’t really know or care," Scot Wingo, CEO of ecommerce analysis company ChannelAdvisor, told Wired. This way, Amazon controls as much of the process as possible, often guaranteeing a degree of efficiency smaller sellers can't promise.

When you order a product "sold by Amazon" but supplied by a third party, Amazon often finds an item that's in storage closest to you, VentureBeat writes. Sometimes the item will even match the one you asked for but will be from a different seller completely, though the first will get credit for the sale. For Amazon, speed is king.

Still, more than 1 billion items sold on Amazon last year came from third-party sellers, adding up to tens of billions of dollars, Amazon told Wired.

Disclosure: Jeff Bezos is an investor in Business Insider through his
personal investment company Bezos Expeditions.